Method and apparatus for zero i.f. frequency and ionospheric pulse reception



United States Patent O s claims. icl. 343-5) This application is a continuation of lapplication No. 291,992 filed by me on July l, 1963 and now abandoned.

This invention relates to a method and apparatus for Zero I.F. `frequency moving target radar and ionospheric pulse reception.

Moving target radar and ionospheric pulse receivers at the present time are conventional superheterodyne receivers with an intermediate frequency in the range from 100 kc./s. to 45 mc/ s. Such receivers require a frequency synthesizer as a local oscillator source having an output frequency which differs from that of the receiver by the LF.

Receivers for such reception have been proposed using a Zero I.F. frequency and are variously known as the Synchrodyne, Homodyne, etc. These receiver designs, however, have in general relied on a steady incoming carrier to phase synchronize the local oscillator and have not been suitable for pulse application.

In` the receiver of this invention, advantage is taken of the fact that moving target radar and ionospheric pulse refiections are time-phase modulated and that no requirement necessarily exists to synchronize the local oscillator to the incoming pulse.

It is an object of this invention to provide a Zero I F.

frequency moving target radar or ionospheric pulse receiver having simplicity and versatility and adaptability for varying nett receiver bandwith by adjusting the cutoff frequency of the pulse amplifier.

It is a further object of the invention to provide a Zero I.F. frequency moving target radar or ionospheric pulse receiver in which no separate frequency synthesizer is required and wherein the transmitter synthesizer-is also used for the local oscillator.

It is another object of this invention to provide a Zero I F. frequency moving target radar or ionospheric pulse receiver whose performance is not appreciably irnpaired by heavy C.W. interference of the type experienced in normal receiver designs.

The invention consists in a moving target radar or ionospheric pulse signal receiver for receiving echo signals comprising pulses of oscillation of which the frequency is known and of which the phase varies as a function of time, depending upon the propagation time from the transmitter to the receiver, comprising means for forming the product of the received pulses and an oscillation of said known frequency, means for selecting from the output of the product forming means the component corresponding to the pulse envelope of the received signal, the amplitude and polarity of said pulse envelope being a function inter alia of the phase variation of said oscillation, means for deriving from said pulse envelope component, rectifying said pulse envelope component to derive uni-directional signal containing excursions of only one polarity, and means for integrating said unidirectional signal over an interval of time sufficient to derive an output signal substantially independent of said variations in pulse envelope amplitude and polarity due to said phase variations.

The invention is illustrated, by way of example, in the accompanying Idrawings in which:

FIGURE 1 is a receiver block diagram of one example of a receiver according to the invention, and

FIGURE 2 shows -how the output of the rectifying amplifier varies in amplitude as a function of qb, the incoming signal phase angle.`

FIGURE 1 shows a block diagram of the Zero I.F. frequency moving target radar or ionospheric pulse receiver. The incoming signal is intercepted by the antenna and passed t-o a balanced product detector 1. For convenience, the incoming signal function is designated P1 and is fed to the balanced product detector 1. The incoming signal P1, is preferably preampli-fied in a tuned R.F. amplifier 2, before being detected. A local oscillator 3 generates a local oscillator signal function P2. The product detector 1 will develop an output sign-al proportional to the product of the incoming signal P1 and the local oscillator signal function P2. The incoming signal P1 and the local oscillator signal function P2 may be conveniently expressed by the following two expressions:

incoming signal function P1=kP sin (wf-l-b) local oscillator function P2=a sin wt where a and k -are constants, P is a pulse envelope function of the incoming signal and is a phase variable depending on the exact propagation time of the ionospheric path.

lIf the output from the balanced product detector 1 is represented by P3, then The high frequency term cos (ZWt-l-qs), will then be removed, by filtering, by filter 8, giving a net output signal designated P4 such that This expression indicates that the signal P4 is an oscillation of frequency depending on the pulse envelope function P and of amplitude dependent on 1/2 ak cos 15, The significant fact is that the amplitude is proportional to cos qb and will pass through zero as cos qb passes through zero, whereas it is Idesired to obtain an output which is independentv of but is otherwise dependent upon the amplitude of the received signal so that information can be derived concerning the reflecting medium or the propagation path of the signal.

The signal now passes through a high gain pulse amplilier 4 to bring its amplitude to a suitable level and wherein the output of the pulse amplifier is designated P5. The output of the pulse amplifier P5 is then fed to a rectifying amplifier 5 which is a linear ampli-fier wherein the positive and negative excursions present in the input signal Iamplitude due to the factor cos will be reproduced as excursions in only one direction. The action is analogous to that of a full wave rectifier circuit wherein a sinusoidal input wave form is converted for instance to a train of positive or negative half sinusoids. This can vary from 0 to a maximum value when qb is varied in increments as shown in FIGURE 2. However, advantage is taken of the fact that will vary in a semirand-om fashion with time on a radar or an ionospheric path, to provide a nett output, P6, proportional to the area under the curve shown in FIGURE 2. Thus P6 is obtained from the rectifying amplifier which integrates return signals from ia succession of pulses over a sufiicient interval of time with respect to variation of cos to yield the desired result of an output independent of the phase p of the signal P1. This result is achieved moreover without the necessity of finding out the relative phase shift undergone by the pulsed carrier oscillation in travelling from the transmitter to the reflecting medium and from the reflecting medium to the receiver, so that the frequency synthesizer for the transmitter carrier oscillation can also be used directly as the local oscillator.

T-he R.F. amplifier 2 and local oscillator 3 may be of stand-ard configuration and common to those skilled in the art. The same applies to the rectifying amplifier 5.

The embodiments f the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A receiver in a pulse echo radar system for receiving echo signals comprising pulses of oscillation of which the frequency is known and of which the phase varies as a function of time, depending upon the propagation time from the transmitter to the receiver, comprising means for forming the product of the received pulses and an oscillation of said known frequency, means for selecting from the output of the product forming means the component corresponding to the pulse envelope of the received signal, the amplitude and polarity of said pulse envelope being a function interalia of the phase variation of said oscillation, means for rectifying said pulse envelope component to derive a uni-directional signal containing eX- cursions of only one polarity, and means for integrating said unidirectional signal over an interval of time sufiicient to derive an output signal substantially independent of said variations in pulse envelope amplitude and polarity due to said phase variations.

2. A receiver according to claim 1 in which said means for rectifying said pulse envelope component and said means for integrating said unidirectional signal comprises a full wave rectifying amplifier which receives said pulse envelope component as said input signal.

3. A receiver according to claim 1 in which the said product forming means comprises a lbalanced product detector to which `are applied the received pulses and an oscillation obtained from the frequency synthesizer which synthesizes the transmitted oscillation.

4. A Zero LF. frequency radar pulse signal receiver for reception of a signal comprising pulse of oscillation of which the phase varies as a function of time, said receiver comprising:

(i) an antenna for receiving said radar pulse signal said pulse signal having a signal function defined by P1, Where P1=kP sin (wt-Hb) wherein P=a pulse envelope function of said signal =a phase variable k=a constant w=the angular frequency of said signal (ii) a local oscillator having an output (P2) characterized by the equation P2=a sin wt wherein a=a constant (iii) a balanced product detector adapted to receive said signal (P1) from said antenna, and said output (P2) from said local oscillator, said detector having an output (P3) which is the product of said signal (P1) from said antenna and said output from said local oscillator, said output from said detector being characterized by the equation,

(iv) a filter adapted to receive said detector output, said filter having an output (P4) wherein said high frequency component is substantially removed, said output P4 being represented by 1/2 akP cos rp,

(v) a zero LF. pulse amplifier adapted to receive said output (P4) from said filter and having an output (P5),

(vi) Ian amplifier adapted to receive said output (P5) and including rectifying means for rectifying said pulse envelope component to derive a uni-directional signal containing excursions of only one polarity and integrating means to integrate said uni-directi-onal signal and yield an output signal substantially independent of the foctor cos gb in the amplitude of the signal P4 but otherwise dependent upon said amplitude.

5. The apparatus of claim 3 including a tuned RF.

pre-amplifier for amplifying said radar pulse signal (P1) before being received by said balanced product detector.

References Cited by the Examiner UNITED STATES PATENTS 2,535,274 12/1950 Dicke 343-77 X 3,132,339 5/1964 Boughnon 343-8 CHESTER L. IUSTUS, Primary Examiner. LEWIS H. MYERS, Examiner. R. D. BENNETT, Assistant Examiner. 

1. A RECEIVER IN A PULSE ECHO RADAR SYSTEM FOR RECEIVING ECHOS SIGNALS COMPRISING PULSES OF OSCILLATION OF WHICH THE FREQUENCY IS KNOWN AND OF WHICH THE PHASE VARIES AS A FUNCTION OF TIME, DEPENDING UPON THE PROPAGATION TIME FROM THE TRANSMITTER TO THE RECEIVER, COMPRISING, MEANS FOR FORMING THE PRODUCT OF THE RECEIVED PULSES AND AN OSCILLATION OF SAID KNOWN FREQUENCY, MEANS FOR SELECTING FROM THE OUTPUT OF THE PRODUCT FORMING MEANS THE COMPONENT CORRESPONDING TO THE PULSE ENVELOPE OF THE RECEIVED SIGNAL, THE AMPLITUDE AND POLARITY OF SAID PULSE ENVELOPE BEING A FUNCTION INTERALIA OF THE PHASE VARIATION OF SAID OSCILLATION, MEANS FOR RECTIFYING SAID PULSE ENVELOPE COMPONENT TO DEVICE AN UNI-DIRECTIONAL SIGNAL CONTAINING EXCURSIONS TO ONLY ONE POLARITY, AND MEANS FOR INTERGRATING SAID UNIDIRECTIONAL SIGNAL OVER AN INTERVAL OF TIME SUFFICIENT TO DERIVE AN OUTPUT SIGNAL SUBSTANTIALLY INDEPENDENT OF SAID VARIATIONS IN PULSE ENVELOPE AMPLITUDE AND POLARITY DUE TO SAID PHASE VARIATIONS. 